1. Field of the Invention
The present invention relates to a latent curing agent for epoxy resin and a curable epoxy resin composition having the same incorporated therein, and in particular to a curing agent for epoxy resin providing storage stability and low-temperature rapid curability to an epoxy resin composition, as well as a curable epoxy resin composition having the same incorporated therein which is excellent in storage stability and has low-temperature rapid curability.
Further, the present invention relates to a latent curing agent for epoxy resin and a curable epoxy resin composition which are suitable for application in the electronics field such as encapsulants for semiconductor, anisotropic conductive films, conducting composites, etc.
2. Background Prior Art
Based on the recent remarkable development in the field of electronics, the density of semiconductor element circuits has been rapidly increased and simultaneously, large-scale production thereof has become feasible, whereby the miniaturization and high-performance of electronic devices are now in rapid progress. Further, as these electronic devices become into wide use, improved workability and reduced costs in large-scale production have got demanded. Further, with respect to epoxy resin used as adhesive for these electronic devices and latent curing agent for such epoxy resin, there is a need for improved high-performance in various physical. properties. As a matter of course, the utilization of epoxy resin is not limited to this.
The epoxy resin composition includes a two-pack type which is used by mixing an epoxy resin as the main agent with a curing agent just before use and a one-pack type having an epoxy resin as the main agent previously mixed with a curing agent. Of the two, the one-pack type is preferably used because it can prevent erroneous formulation and enables machinery automation on line. For the one-pack type epoxy resin composition; is needed a so-called latent curing agent which does not react with the epoxy resin compound at room temperature, and is caused to react and cure upon heating.
As for the latent curing agent, some curing agents have been proposed so far, and typical examples thereof include dicyandiamide, dibasic acid dihydrazide, boron trifluoride amine complex salt, guanamines, melamine, imidazoles, etc. However, an epoxy resin composition resulting from mixing an epoxy compound with dicyandiamide, melamine or a guanamine is excellent in storage stability, but is disadvantageous in that it requires, for curing, a long time at high temperatures of 150.degree. C. or more. Further, such epoxy resin composition is often used in combination with a curing accelerator to reduce curing time. The addition of a curing accelerator indeed leads to a reduction in the curing time of the curing epoxy resin composition but causes such problem that the storage stability thereof is significantly deteriorated. The epoxy resin compositions having a dibasic acid dihydrazide or an imidazole incorporated therein as the latent curing agent, are cured at relatively low temperatures but poor in storage stability. The boron trifluoride amine complex is highly hygroscopic and exerts adverse effects on various characteristics of a cured product from an epoxy resin composition having said complex incorporated therein. In these circumstances, a latent curing agent for epoxy resin, which gives an epoxy resin composition excellent in storage stability with low-temperature rapid curability, is highly desired.
To improve or solve these problems, JP 56-155222A and JP 57-100127A propose curing agents wherein dialkyl amines are addition-reacted with epoxy compounds, and JP 59-53526A proposes curing agents wherein an aminoalcohol or aminophenol is addition-reacted with an epoxy compound. Further, U.S. Pat. Nos. 406,625 and 4,268,656 propose curing agents wherein imidazole compounds or N-methyl piperazine is addition-reacted with epoxy compounds at their secondary amino groups.
However, the curing agents described above do not sufficiently confer storage stability and low-temperature curability on epoxy resin compositions in which they have been incorporated. In particular, those epoxy resin compositions wherein bisphenol F type epoxy compounds or reaction diluents such as monoepoxy compounds or diepoxy compounds have been used, do not exhibit satisfactory storage stability.
Further, phenol resin, dicyandiamide, hydrazide compounds or the like have been used heretofore as a latent curing agent, but they require, for curing, heating treatment in an oven at 150 to 200.degree. C. for a long time. Recently, there is a need for rapidly curable resin compositions which can be cured in a short time, with the view of improving workability if the step of assembling semiconductor devices and reducing manufacturing costs. In addition, as circuits become highly densified on printed circuit boards, etc., a reduction in accuracy due to thermal stress upon high-temperature curing, has been worried about, and also from the viewpoint of energy costs, there has been a need for curing at relatively low temperatures to minimize the thermal history of printed circuit boards, etc.
As typical examples of latent curing agents with low-temperature rapid curing properties, e.g. amine adduct type latent curing agents are known and JP 9-92029A, for example, describes an example where a commercial latent curing agent "Ajicure" (a product of Ajinomoto Co., Ltd.) is used as an conductive paste.
On the other hand, as semiconductor production technology rapidly advances, it has become impossible to ignore the physical properties of a curing agent per se incorporated in curable epoxy resin compositions as well as its effect on electrical reliability, and there has been a need for latent curing agents more suitable in the field of electronics than those conventionally used.